Trivalent chromium plating solution and trivalent chromium  plating method using same

ABSTRACT

A trivalent chromium plating solution that does not cause the problems including the occurrence of deposition failure of the plating and color unevenness, such as a brown stripe pattern, in the plating even though a metal impurity is incorporated into the plating solution, and a trivalent chromium plating method are provided by a trivalent chromium plating solution containing a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, further containing an unsaturated sulfonic acid compound represented by the following general formula (1) (wherein in the formula (1), R 1  represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R 2  represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal), and a trivalent chromium plating method using the same. 
       R 1 —CH═CH—R 2 —SO 3 X  (1)

TECHNICAL FIELD

The present invention relates to a trivalent chromium plating solution and a trivalent chromium plating method using the same.

BACKGROUND ART

Chromium plating is used as a coating film for decoration owing to the silvery white color thereof. Hexavalent chromium has been used for the chromium plating, but the use of hexavalent chromium is restricted in recent years due to the environmental implications thereof, and the technology is shifted to the use of trivalent chromium.

Many reports have been made for the technique using trivalent chromium by various manufacturers (see, for example, PTL 1).

However, the trivalent chromium plating solution has problems including the occurrence of deposition failure of the plating and color unevenness, such as a brown stripe pattern, in the plating caused by the incorporation of metal impurities derived from the first plating and the chemicals and the like associated with the plating.

CITATION LIST Patent Literature

PTL 1: JP-A-2009-74170

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a trivalent chromium plating solution that does not cause the problems including the occurrence of deposition failure of the plating and color unevenness, such as a brown stripe pattern, in the plating even though metal impurities are incorporated into the plating solution.

Solution to Problem

As a result of the earnest investigations by the present inventors, it has been found that the problems including the occurrence of deposition failure of the plating and color unevenness, such as a brown stripe pattern, in the plating can be prevented from occurring even through metal impurities are incorporated into the plating solution, by using an unsaturated sulfonic acid compound having a particular structure contained in a trivalent chromium plating solution containing a chloride as a conductive salt, and thus the present invention has been completed. Furthermore, it has also been found that not only the aforementioned problems, but also burnt deposit in a high current density in plating can be prevented from occurring, by positively using nickel as a metal impurity contained in the trivalent chromium plating solution, and thus the present invention has been completed.

Accordingly, the present invention relates to a trivalent chromium plating solution containing a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, further containing an unsaturated sulfonic acid compound represented by the following general formula (1):

R₁—CH═CH—R₂—SO₃X  (1)

wherein in the formula (1), R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal.

The present invention also relates to a trivalent chromium plating method including electroplating an article to be plated with the trivalent chromium plating solution.

The present invention further relates to a method for enhancing resistance of a trivalent chromium plating solution containing a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, against metal impurities contained, including using an unsaturated sulfonic acid compound represented by the following general formula (1):

R₁—CH═CH—R₂—SO₃X  (1)

wherein in the formula (1), R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal, further contained in the trivalent chromium plating solution.

The present invention still further relates to a method for preventing burnt deposit in a high current density in plating with a trivalent chromium plating solution containing a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, including using an unsaturated sulfonic acid compound represented by the following general formula (1), and nickel:

R₁—CH═CH—R₂—SO₃X  (1)

wherein in the formula (1), R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal, further contained in the trivalent chromium plating solution.

The present invention still more further relates to a chromium-plated product including an article to be plated, electroplated with the trivalent chromium plating solution.

Advantageous Effects of Invention

The trivalent chromium plating solution of the present invention is an excellent one that does not cause the problems including the occurrence of deposition failure of the plating and color unevenness, such as a brown stripe pattern, in the plating even through metal impurities are incorporated into the plating solution.

Furthermore, not only the aforementioned problems, but also burnt deposit in a high current density in plating can be prevented from occurring by positively using nickel as a metal impurity contained in the trivalent chromium plating solution of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration showing the position where the throwing distance is measured in the Hull cell test of Example 1.

FIG. 2 is an image showing the Hull cell appearance of the nickel-plated brass plate (with burnt deposit) after plating with the plating solution having no nickel added in Example 4.

FIG. 3 is an image showing the Hull cell appearance of the nickel-plated brass plate (without burnt deposit) after plating with the plating solution having 10 ppm of nickel added in Example 4.

DESCRIPTION OF EMBODIMENTS

The trivalent chromium plating solution of the present invention (which may be hereinafter referred to as a “plating solution of the present invention”) is a trivalent chromium plating solution that contains a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, and further contains an unsaturated sulfonic acid compound represented by the following general formula (1).

R₁—CH═CH—R₂—SO₃X  (1)

In the formula (1), R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal, and it is preferred that R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 6 or hydrogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 6; and X represents hydrogen, sodium, or potassium. Specific examples of the unsaturated sulfonic acid compound represented by the formula (1) include sodium vinylsulfonate, sodium allylsulfonate, sodium p-styrenesulfonate, and sodium β-styrenesulfonate. The unsaturated sulfonic acid compound may be used alone or as a combination of two or more kinds thereof. The content of the unsaturated sulfonic acid compound in the plating solution of the present invention is not particularly limited, and for example, is from 0.01 to 20 g/L, and preferably from 0.1 to 5 g/L.

The trivalent chromium compound used in the plating solution of the present invention is not particularly limited, examples of which include basic chromium sulfate, chromium sulfate, chromium chloride, chromium sulfamate, and chromium acetate, and basic chromium sulfate, chromium sulfate, and chromium chloride are preferred. The trivalent chromium compound may be used alone or as a combination of two or more kinds thereof. The content of the trivalent chromium compound in the plating solution of the present invention is not particularly limited, and for example, is from 1 to 20 g/L, and preferably from 5 to 15 g/L, in terms of metallic chromium.

The conductive salt used in the plating solution of the present invention is a chloride. The kind of the chloride is not particularly limited, examples of which include potassium chloride, ammonium chloride, and sodium chloride. The chloride may be used alone or as a combination of two or more kinds thereof. The content of the chloride in the plating solution of the present invention is not particularly limited, and for example, is from 150 to 400 g/L, and preferably from 200 to 350 g/L.

The pH buffering agent used in the plating solution of the present invention is not particularly limited, examples of which include boric acid, sodium borate, potassium borate, phosphoric acid, and dipotassium hydrogen phosphate, and boric acid and sodium borate are preferred. The pH buffering agent may be used alone or as a combination of two or more kinds thereof. The content of the pH buffering agent in the plating solution of the present invention is not particularly limited, and for example, is from 10 to 150 g/L, and preferably from 50 to 100 g/L.

The complexing agent used in the plating solution of the present invention is not particularly limited, examples of which include formic acid, ammonium formate, potassium formate, citric acid, and triammonium citrate. Among these, ammonium formate and triammonium citrate are preferred. The complexing agent may be used alone or as a combination of two or more kinds thereof. The content of the complexing agent in the plating solution of the present invention is not particularly limited, and for example, is from 0.3 to 2 times by mol, and preferably from 0.8 to 1.5 times by mol, the metallic chromium concentration.

The plating solution of the present invention may further contain ammonium bromide, potassium bromide, and the like.

The pH of the plating solution of the present invention is not particularly limited, as far as the solution is acidic, and for example, is preferably from 2 to 4, and more preferably from 2.5 to 3.5.

The preparation method of the plating solution of the present invention is not particularly limited, and for example, the solution can be prepared by adding and mixing the trivalent chromium compound, the chloride salt, the pH buffering agent, the complexing agent, and the unsaturated sulfonic acid compound with water at from 40 to 50° C., adding and mixing the other components therewith depending on necessity, and controlling the pH thereof.

The plating solution of the present invention does not cause the problems including the occurrence of deposition failure of the plating and color unevenness, such as a brown stripe pattern, in the plating even though metal impurities are contained therein (i.e., has resistance against metal impurities). In particular, the plating solution of the present invention does not cause the problems even though a large amount of metal impurities are contained in a concentration of approximately several hundred ppm during the long-term use or suddenly. The metal impurities herein are metals derived from the first plating and the chemicals and the like associated with the plating. Specific examples of the metals include nickel, zinc, copper, and hexavalent chromium, and preferably include nickel and copper, which are frequently used as the first plating.

In the case where the plating solution of the present invention positively uses nickel as a metal impurity contained therein, burnt deposit in a high current density in plating can be prevented from occurring. The high current density herein means portions where the current is concentrated, such as a corner part and an apex part of an article. Nickel that can be used in the plating solution of the present invention is not particularly limited, and examples thereof include a nickel salt, such as nickel chloride and nickel sulfate. The content of nickel in the plating solution of the present invention is not particularly limited, and for example, is from 10 to 500 ppm, preferably from 15 to 200 ppm, and more preferably from 20 to 100 ppm.

Nickel can prevent burnt deposit in a high current density from occurring also in plating with a trivalent chromium plating solution containing a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, obtained by excluding the unsaturated sulfonic acid compound represented by the general formula (1) from the plating solution of the present invention, at the same concentration. The kinds and the concentrations of the trivalent chromium compound, the chloride, the pH buffering agent, and the complexing agent may be the same as in the plating solution of the present invention.

An ordinary trivalent chromium plating solution contains iron or cobalt for the enhancement of the throwing power for a low current density, but the plating solution of the present invention can have enhanced throwing power without the addition of iron and/or cobalt. A plating solution containing iron or cobalt has a tendency that the corrosion resistance of the plating film is decreased due to codeposition of iron or cobalt therein. Accordingly, it is preferred that the plating solution of the present invention contains substantially no iron and/or cobalt. The plating solution of the present invention that contains substantially no iron and/or cobalt means that the content of iron and/or cobalt is 2 ppm or less, preferably 1 ppm or less, and more preferably 0.5 ppm or less. The amount of iron and/or cobalt can be analyzed by the ICP-MS method, the atomic absorption spectrometry, or the like.

Chromium plating can be formed on an article to be plated by electroplating the article to be plated with the plating solution of the present invention in the same manner as for the ordinary chromium plating solution.

The condition of the electroplating is not particularly limited, and for example, the electroplating may be performed at a bath temperature of from 25 to 45° C., with carbon or iridium oxide as an anode, at a cathode current density of from 4 to 20 A/dm², for a period of from 1 to 15 minutes.

Examples of the article to be plated capable of being electroplated include a metal, such as iron, stainless steel, and brass, and a resin, such as ABS and PC/ABS. The article to be plated may be subjected to a treatment, such as copper plating and nickel plating, in advance before the treatment with the plating solution of the present invention.

The chromium plating thus obtained becomes chromium plating having an appearance, throwing power, and a deposition rate that are equivalent to the use of hexavalent chromium. The unsaturated sulfonic acid compound represented by the formula (1) can suppress the color tone of the resulting chromium plating from becoming dark, as compared to other unsaturated sulfonic acid compounds. Accordingly, the chromium-plated product is favorably applied to automobile exterior components, such as a door handle and an emblem, and components of an accessory, a faucet, a tool, and the like.

In the case where the plating solution of the present invention contains substantially no iron and/or cobalt, the resulting chromium-plated product also contains substantially no iron and/or cobalt. The chromium-plated product of the present invention that contains substantially no iron and/or cobalt means that the content of iron and/or chromium in the chromium plating is less than 0.5% by atom, and preferably 0.3% by atom or less. The amount of iron and/or cobalt can be analyzed by EDS, XPS, or the like.

EXAMPLES

The present invention will be described in detail with reference to examples and comparative examples below, but the present invention is not limited to the examples.

Example 1 Chromium Plating

The compounds shown in the following basic formulation and Table 1 each were dissolved in water to prepare a trivalent chromium plating solution. The trivalent chromium plating solution was subjected to the Hull cell test using a brass plate having nickel plating formed thereon. The condition of the Hull cell test was a current of 5 A and a plating time of 3 minutes. After plating, the distance of the deposition of the plating film from the left end of the brass plate was measured as shown in FIG. 1, and the throwing power decreasing rate was calculated in the manner described later. The color tone and the appearance were evaluated in the manners described later. The color tone after plating was evaluated in terms of the L* value with a color-difference meter (produced by Konica Minolta, Inc.). The results are shown in Table 1.

<Basic Formulation 1>

Basic chromium sulfate 64 g/L Ammonium formate 16 g/L Potassium chloride 165 g/L Ammonium chloride 100 g/L Ammonium bromide 6 g/L Boric acid 67 g/L

<Basic Formulation 2>

Basic chromium sulfate 64 g/L Ammonium formate 16 g/L Sodium chloride 75 g/L Potassium chloride 165 g/L Ammonium chloride 100 g/L Ammonium bromide 6 g/L Boric acid 67 g/L

<Basic Formulation 3>

Basic chromium sulfate 64 g/L Ammonium tartarate 30 g/L Potassium sulfate 150 g/L Ammonium sulfate 20 g/L Boric acid 80 g/L

TABLE 1 Throwing power Basic Addition Evaluation of Evaluation of decreasing rate formulation Compound concentration L* color tone appearance (%) Example 1 Na vinylsulfonate 25% aqueous solution 1 mL/L 79.54 good good 0.70 product 1 Na vinylsulfonate 25% aqueous solution 5 mL/L 79.52 good good 1.41 1 Na allylsulfonate 36% aqueous solution 0.5 mL/L 78.39 good good not decreased 1 Na p-styrenesulfonate 1.5 g/L 79.23 good good not decreased 2 Na β-styrenesulfonate 1.0 g/L 79.61 good good 0.70 2 Na vinylsulfonate 25% aqueous solution 1 mL/L 79.48 good good not decreased Comparative 1 not added — 78.63 good good 0 (standard) product 1 Na 2-propine-1-sulfonate 20% aqueous 0.1 mL/L 73.69 poor good 11.27  solution 1 thiodiglycolic acid 1.0 g/L 76.77 poor good 5.63 1 thiourea 30 mg/L 75.70 poor good 7.75 1 1-allyl-2-thiourea 50 mg/L 75.60 poor good 1.41 1 Na 3-mercapto-1-propanesulfonate 50 mg/L 77.22 poor good 2.11 1 Na 2-hydroxyethanesulfonate 20% 20 mL/L 79.04 good good 1.41 aqueous solution 1 Na saccharate dihydrate 50 mg/L 77.66 poor good 1.41 2 not added — 78.87 good good 0 (standard) 3 not added — 79.68 good good 0 (standard) 3 Na allylsulfonate 36% aqueous solution 1.5 mL/L 80.38 good good not decreased 3 Na vinylsulfonate 25% aqueous solution 1.0 mL/L 73.47 poor good not decreased

<Calculation Method of Throwing Power Decreasing Rate>

(decreasing rate (%))=((standard value−test value))/(standard value)×100

In the expression, the standard value is a value of throwing power with no compound added, and the test value is the measured value in the test under the condition. The values of the standard value are 71 mm for the basic formulation 1, 73 mm for the basic formulation 2, and 74 mm for the basic formulation 3.

<Evaluation of Color Tone> (Evaluation) (Content)

good: L* of 78 or more

poor: L* of less than 78

<Evaluation of Appearance>

good: uniform appearance and no uneven pattern

poor: uneven appearance or uneven pattern

It was found from the results that the unsaturated sulfonic acid compound contained in the plating solution of the present invention was not a substance that the use thereof deteriorated the color tone, the appearance, and the throwing power of the chromium plating.

The contents of iron and cobalt in the trivalent chromium plating solutions each were less than 0.5 ppm, as measured by the ICP-MS method. The contents of iron and cobalt in the resulting chromium plating each were less than 0.3% by atom, as measured by the EDS elemental analysis.

Example 2 Chromium Plating in Presence of Impurity

A trivalent chromium plating solution was prepared in the same manner as in Example 1 except that a Watts solution was added in an amount that provided 100 ppm of nickel as a metal impurity. The trivalent chromium plating solution was subjected to the same test as in Example 1. The results are shown in Table 2.

TABLE 2 Throwing power Basic Addition Evaluation of Evaluation of decreasing rate formulation Compound concentration L* color tone appearance (%) Note Example 1 Na vinylsulfonate 25% 1 mL/L 78.97 good good 4.93 — product aqueous solution 1 Na vinylsulfonate 25% 5 mL/L 79.83 good good 4.23 — aqueous solution 1 Na allylsulfonate 36% 0.5 mL/L 78.04 good good 1.41 — aqueous solution 1 Na p-styrenesulfonate 1.5 g/L 79.01 good good 3.52 — 2 Na β-styrenesulfonate 1.0 g/L 79.22 good good 2.82 — 2 Na vinylsulfonate 25% 1 mL/L 78.99 good good 4.23 — aqueous solution Comparative 1 not added — 77.81 poor poor 6.34 brown stripe product pattern 1 Na 2-propine-1-sulfonate 0.1 mL/L * poor poor — mostly 20% aqueous solution deposition failure 1 thiodiglycolic acid 1.0 g/L 76.48 poor good 7.75 — 1 thiourea 30 mg/L 75.3  poor good 11.27  — 1 1-allyl-2-thiourea 50 mg/L 76.08 poor good 4.23 — 1 Na 3-mercapto-1- 50 mg/L 77.54 poor good 5.63 — propanesulfonate 1 Na 2-hydroxyethanesulfonate 20 mL/L 78.53 poor poor 7.75 — 20% aqueous solution 1 Na saccharate dihydrate 50 mg/L 73.67 poor poor 9.86 brown stripe pattern 2 not added — 77.59 poor poor 6.85 brown stripe pattern 3 not added — * * * 50% or more mostly deposition failure 3 Na allylsulfonate 36% 1.5 mL/L * * * 50% or more mostly aqueous solution deposition failure 3 Na vinylsulfonate 25% 1.0 mL/L * * * 50% or more mostly aqueous solution deposition failure * unmeasurable or unevaluable

It was found that the plating capability of the trivalent chromium plating solution of the present invention (i.e., the example product), which was a chloride bath and contained the unsaturated sulfonic acid compound having the particular structure, was not influenced by 100 ppm of nickel contained as a metal impurity in the plating solution. It was also found that irrespective of the same trivalent chromium plating solution, the sulfuric acid bath (i.e., the comparative product having the basic formulation 3) had no resistance against metal impurities even though the unsaturated sulfonic acid compound having the particular structure was contained. It was further found that irrespective of the chloride bath, in the case where the particular structure was not used, the plating capability was influenced by 100 ppm of nickel contained as a metal impurity in the plating solution.

The contents of iron and cobalt in the trivalent chromium plating solutions each were less than 0.5 ppm, as measured by the ICP-MS method. The contents of iron and cobalt in the resulting chromium plating each were less than 0.3% by atom, as measured by the EDS elemental analysis.

Example 3 Chromium Plating in Presence of Impurity

A trivalent chromium plating solution containing the compounds shown in the basic formulation 1 used in Example 1 and the Table 3 below, and a copper chloride aqueous solution in an amount that provided 20 ppm of copper as a metal impurity was prepared. The trivalent chromium plating solution was subjected to the same test as in Example 1. The results are shown in Table 3. For comparison, a trivalent chromium plating solution containing no unsaturated sulfonic acid compound having the particular structure was subjected to the same test. The results are also shown in Table 3.

TABLE 3 Throwing power Basic Addition Evaluation of Evaluation of decreasing rate formulation Compound concentration L* color tone appearance (%) Note Example 1 Na vinylsulfonate 25% 2 mL/L 79.39 good good 0.47 — product aqueous solution Comparative 1 not added — 79.14 good poor 2.82 white pattern product

It was found that the plating capability of the trivalent chromium plating solution of the present invention (i.e., the example product) was not influenced by 20 ppm of copper contained as a metal impurity in the plating solution.

The contents of iron and cobalt in the trivalent chromium plating solutions each were less than 0.5 ppm, as measured by the ICP-MS method. The contents of iron and cobalt in the resulting chromium plating each were less than 0.3% by atom, as measured by the EDS elemental analysis.

Example 4 Chromium Plating

The compounds shown in the following basic formulation 4, a sodium vinylsulfonate 25% aqueous solution, and nickel (added in the form of nickel chloride) were dissolved in water in the concentrations shown in Table 4 to prepare a trivalent chromium plating solution. The trivalent chromium plating solution was subjected to the Hull cell test using a brass plate having nickel plating formed thereon. The condition of the Hull cell test was a current of 5 A and a plating time of 3 minutes. The trivalent chromium plating solution was subjected to the same test as in Example 1. The presence of burnt deposit in a high current density (which was the left end portion in FIG. 1) was visually evaluated. The results are shown in Table 4. For showing an example of the presence of burnt deposit, the brass plate after plating with a plating solution containing no nickel (with burnt deposit) is shown in FIG. 2, and the appearance of the brass plate after plating with a plating solution containing 10 ppm of nickel (without burnt deposit) is shown in FIG. 3.

<Basic Formulation 4>

Basic chromium sulfate 64 g/L Ammonium formate 16 g/L Sodium chloride 70 g/L Potassium chloride 140 g/L Ammonium chloride 85 g/L Ammonium bromide 6 g/L Boric acid 67 g/L

TABLE 4 Basic Na vinylsulfonate 25% Evaluation of Evaluation of Burnt deposit in high formulation aqueous solution Nickel L* color tone appearance current density Example 4 3 mL/L 10 ppm 79.57 good good none product 4 3 mL/L 30 ppm 80.42 good good none 4 3 mL/L 50 ppm 80.28 good good none 4 5 mL/L 300 ppm 79.91 good good none 4 5 mL/L 500 ppm 79.57 good good none Comparative 4 3 mL/L 0 ppm 80.27 good good found product

It was found that burnt deposit in a high current density in plating was prevented from occurring, by positively using nickel as a metal impurity contained in the trivalent chromium plating solution of the present invention (i.e., the example product).

The contents of iron and cobalt in the trivalent chromium plating solutions each were less than 0.5 ppm, as measured by the ICP-MS method. The contents of iron and cobalt in the resulting chromium plating each were less than 0.3% by atom, as measured by the EDS elemental analysis.

Reference Example 1 Chromium Plating

To a plating solution having the basic formulation 4, a sodium vinylsulfonate 25% aqueous solution was not added, but nickel was added in an amount of 0, 10, 30, or 50 ppm, so as to prepare a trivalent chromium plating solution. The trivalent chromium plating solution was subjected to the Hull cell test using a brass plate having nickel plating formed thereon. The condition of the Hull cell test was a current of 5 A and a plating time of 3 minutes. The trivalent chromium plating solution was subjected to the same test as in Example 1 and the same evaluation of the presence of burnt deposit in a high current density as in Example 4.

It was found that by positively using nickel as a metal impurity contained in the trivalent chromium plating solution of the present invention, the color tone and the appearance of the resulting plating were equivalent to the case where plating was formed with the solutions having a sodium vinylsulfonate 25% aqueous solution added thereto in Examples 1 to 3, and burnt deposit in a high current density in plating was prevented from occurring. In the case where nickel was not added, burnt deposit in a high current density in plating occurred.

INDUSTRIAL APPLICABILITY

The trivalent chromium plating solution of the present invention can be applied to various purposes as similar to plating with hexavalent chromium. 

1. A trivalent chromium plating solution, comprising: a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, a complexing agent, and an unsaturated sulfonic acid compound of formula (1): R₁—CH═CH—R₂—SO₃X  (1) wherein R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal.
 2. The trivalent chromium plating solution according to claim 1, wherein the unsaturated sulfonic acid compound is at least one selected from the group consisting of sodium vinylsulfonate, sodium allylsulfonate, sodium p-styrenesulfonate, and sodium β-styrenesulfonate.
 3. The trivalent chromium plating solution according to claim 1, wherein the chloride is at least one selected from the group consisting of potassium chloride, ammonium chloride, and sodium chloride.
 4. The trivalent chromium plating solution according to claim 1, further comprising: a metal impurity.
 5. The trivalent chromium plating solution according to claim 1, further comprising: nickel.
 6. The trivalent chromium plating solution according to claim 1, wherein the trivalent chromium plating solution comprises substantially no iron and/or cobalt.
 7. A trivalent chromium plating method, comprising: electroplating an article with the trivalent chromium plating solution according to claim
 1. 8. A method for enhancing resistance of a trivalent chromium plating solution against metal impurities contained in the trivalent chromium plating solution, the method comprising: adding an unsaturated sulfonic acid compound in the trivalent chromium plating solution, wherein the trivalent chromium plating solution comprises a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, and the unsaturated sulfonic acid compound is a compound of formula (1): R₁—CH═CH—R₂—SO₃X  (1) wherein R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal.
 9. A method for preventing burnt deposit in a high current density in plating with a trivalent chromium plating solution, the method comprising: adding an unsaturated sulfonic acid and nickel in the trivalent chromium plating solution, wherein the trivalent chromium plating solution comprises a trivalent chromium compound, a chloride as a conductive salt, a pH buffering agent, and a complexing agent, and the unsaturated sulfonic acid compound is a compound of formula (1): R₁—CH═CH—R₂—SO₃X  (1) wherein R₁ represents a hydrocarbon group having a number of carbon atoms of from 1 to 10, hydrogen, or a halogen; R₂ represents nothing or a hydrocarbon group having a number of carbon atoms of from 1 to 10; and X represents hydrogen or an alkali metal.
 10. A chromium-plated product comprising: an article, electroplated with the trivalent chromium plating solution according to claim
 1. 11. The chromium-plated product according to claim 10, wherein the chromium plating comprises substantially no iron and/or cobalt. 